1. Field of the Invention
The invention relates to ballistic armor. More particularly the invention relates to ballistic armor made by cold gas dynamic spraying a coating of hard particles onto an aluminum armor plate followed by friction stirring the coating into the aluminum armor plate.
2. Discussion of the Related Art
Ballistic armor made of aluminum alloy is used by the military for applications for which excess weight limits usefulness. One application is the aluminum alloy armor on armored personnel carriers and other military vehicles for which the carrying capacity is limited by weight. Another application is the hulls and armor on small combatant craft.
Use of ballistic grade aluminum has increased due to the ballistic performance of this low density metal when used as primary armor on combat vehicles and support vehicles. These materials are resistant to small arms projectiles and explosion fragments. Ballistic grade aluminum armor is also resistant to explosive blasts and multiple fragment impacts from improvised explosive devices. An aluminum alloy of interest is the alloy 5083 in both wrought plate (MIL-DTL-46027J) and extruded form (MIL-DTL-46083D). Alloy 5083 non-heat treatable wrought aluminum derives its properties from strain-hardening by rolling. Other aluminum alloy armors have been investigated.
Some aluminum armor alloys in use are alloy 5083 and alloy 5456 meeting the requirements of U.S. Military Specification MIL-A-46027J (MR), and alloy 7039 meeting the requirements of U.S. Military Specification MIL-A-46063H (MR).
The primary wrought aluminum alloy used in U.S. combat vehicles is aluminum-magnesium alloy 5083 defined by mechanical properties, corrosion requirements and ballistic standards specified in MIL-DTL-46027J (MR). The ballistic, standard is similar for related alloy 5456 that is used primarily in naval applications. The specification sets the minimum ballistic requirements for thicknesses from 6.35 mm (0.25 inch) to 76.2 mm (3.00 inch) that must be met to be certified under the military standard. The primary strengthening method for commercial grade alloy 5083 is strain hardening by rolling and tempering. The main strain-hardened tempered alloys are 5083-H131 and 5083-H321.
Some commercially available heat-treatable aluminum alloys used for appliqué do not have a military specification for ballistics. Aluminum-silicon-magnesium alloy 6061 with 16 heat treating is referred to as aluminum alloy 6061-16. Other alloys include aluminum-copper alloy 2024-1351 and aluminum-zinc alloy 7075-T6. Alloy 7075 is similar to alloy 7039, but higher in strength. There has also been interest in the commercially available aluminum alloy Alustar 5059-H131 which is in the same family as the 5083/5456 alloys.
Amorphous metals, referred to herein as metallic glass, have remarkable physical properties. These amorphous metals and alloys have a non-crystalline high-temperature amorphous structure at room temperature. The more common microstructure of these metals is a polycrystalline solid at room temperature.
Amorphous metals are formed by cooling a liquid melt very rapidly to avoid formation of the polycrystalline state. Rapid quenching circumvents the finite amount of time required to generate a solidified, polycrystalline phase morphology. For this reason amorphous metals are normally found in the form of powders, fibers, or thin ribbons. The high surface-to-volume ratio of these forms allows rapid cooling of the molten metal and retention of the amorphous phase which is retained by the lack of atomic mobility at room temperatures. There are also some special amorphous metal alloys that contain many variable atomic sizes which retard crystalline formation and can be fabricated in thicker, centimeter size dimensions.
Because amorphous alloys do not have grain boundaries, dislocations, or other typical polycrystalline features, they frequently demonstrate very high hardness and strength. For some metallic glasses the strength is on the order of 250 ksi (1700 MPa). In some compositions there is a high co-efficient of restitution demonstrating rapid rebound between loading and unloading with nearly complete recovery.
While amorphous metals exhibit a number of remarkable properties including hardness, strength and restitution; they also exhibit much less ductility under certain kinds of loading and can shatter at high impact. One possibility of taking advantage of their mechanical and electromagnetic capabilities and avoiding the shattering problem is to embed them in a more ductile matrix.
Explosive compaction has been tried to embed metallic glass in aluminum or aluminum matrices. However, materials fabricated by compaction contain a greater number of internal defects because the particulates that form the compacted material are not in contact on all surfaces as would occur in a fluid environment that promotes wetting and mixing of the components. A compacted structure, though improved over a more brittle glassy phase, still exhibits rather low shatter resistance. Also the explosive process has limitations due to pulverizing the metallic glass at higher explosive compaction levels. Rather high hardness values for compacted material of up to about Rockwell hardness 65 were obtained with incorporation of 16 volume % metallic glass.
Amorphous metals have been mixed into kinetic energy penetrators by extrusion in an effort to replace depleted uranium. The strength and self-sharpening characteristics of the amorphous metals were useful for this application. The process was expensive and included a very dense, high cost amorphous metal comprising zirconium, columbium, nickel, copper and aluminum.
I. Boromel, L. Ceschini, A. Morri and G. L. Garagnani, “Friction Stir Welding of Aluminum Based Composites Reinforced with A1203 Particles: Effects on Microstructure and Chirpy Impact Energy”, Metallurgical Science and Technology, vol. 24. No. 1 (1983) reports that aluminum alloy 6061 was impregnated with 20 volume % microscopic alumina (Al2O3) particles by high energy mixing of the alumina powder in molten aluminum before casting. This was followed by T6 heat treatment.
There is a continuing need in the art for improvements in ballistic aluminum armor.